U.S. Pat. No. 4,284,686 (Wedding) describes a series of ophthalmic filter lenses and their production. These lenses are specially designed to alleviate the discomfort experienced in bright light by individuals afflicted with certain visual deficiencies.
All of the commercially important photochromic glasses are glasses which contain a precipitated, microcrystalline, silver halide phase. It is this phase which is considered to cause the reversible darkening of the glass upon exposure to light. U.S. Pat. No. 3,208,860 (Armistead et al.) provides the basic description of this family of glasses. Subsequent work has resulted in the development of many new families of photochromic glasses exhibiting faster darkening and/or fading response. U.S. Pat. No. 4,190,451 (Hares et al.), for example, provides a description of some recently developed photochromic glasses of this type.
The patent discloses glasses which are particularly suitable for use in the inventive method. Such glasses consist essentially, expressed in weight percent on the oxide bases, of about 0-2.5% Li.sub.2 O, 0-9% Na.sub.2 O, 0-17% K.sub.2 O, 0-6% Cs.sub.2 O, 8-20% Li.sub.2 O+Na.sub.2 O+K.sub.2 O+Cs.sub.2 O, 14-23% B.sub.2 O.sub.3, 5-25% Al.sub.2 O.sub.3, 0-25% P.sub.2 O.sub.5, 20-65% SiO.sub.2, 0.004-0.02% CuO, 0.15-0.3% Ag, 0.1-0.25% Cl, and 0.1-0.2% Br, wherein the molar ratio of alkali metal oxides:B.sub.2 O.sub.3 varies between about 0.55-0.85 and the weight ratio Ag:(Cl+Br) ranges between about 0.65-0.95. Those glasses may also optionally contain up to about 10% total of other ingredients selected from the group in the indicated proportions of 0-6% ZrO.sub.2, 0-3% TiO.sub.2, 0-0.5% PbO, 0-7% BaO, 0-4% CaO, 0-3% MgO, 0-6% Nb.sub.2 O.sub.5, 0-4% La.sub.2 O.sub.3 and 0-2% F. Finally, those compositions are compatible with the conventional glass colorants selected from the transition metal oxides and rare earth metal oxides. Hence, up to about 1% total of transition metal oxide colorants and/or up to 5% total of rare earth metal oxide colorants may be included to modify the color of the bulk glass.
Colored, ophthalmic lenses, developed in accordance with the Wedding patent teachings, have provided relief for patients having light or glare sensitivity problems. Dye-impregnated, plastic lenses have been developed as alternatives. The latter are sometimes referred to as "blockers" since they are stated to absorb all of the light below a certain wavelength.
A major problem with the "blocker" lens is that total absorption of part of the spectrum greatly distorts color perception. This may also occur in the surface colored, glass lens with an unduly long treating time. However, the time of the reducing treatment may be adjusted so that a carefully controlled, small amount of blue transmission, referred to as a "blue leak," occurs. This provides a less severe distortion of color perception.
Nevertheless, it would be desirable to further improve natural color perception in a protective filter lens.
Subsequent developments have enabled the filter lenses disclosed in the Wedding--686 patent to be produced with much shorter firing times. For example, a filter lens, having its cutoff over a wavelength range of about 450-500 nm. in the visible, can be produced by firing the lens in flowing hydrogen for two hours at about 476.degree. C.
However, it is still necessary to "front surface" the lens after firing. This involves removing the reduced glass from the front surface of the lens. This is necessary to permit access of actinic radiation to darken the photochromic glass. Further, if a fused, multifocal lens is to be produced, it is necessary to remove the reduced layer in order to fuse the segment in place.
One object of the present invention is to obviate the need for the front surfacing procedure.
Another object is to provide a protective filter lens that closely approximates transmission of a natural color scene, that is, allows a viewer to see the actual, undistorted colors in a scene.
A further object is to provide these features either in a lens that is untinted, or in one that has a fixed tint imparted to the glass.
It is also an object to reduce the time factor in the process without impairing the effectiveness of the lens.
Another object is to enable processing of photochromic, progressive lenses.